﻿If the space boundary is physical, it shall be provided by an element (i.e. excluding a virtual element). If the space boundary is virtual, it shall either have a virtual element or an opening providing the space boundary. If the space boundary <em>PhysicalOrVirtualBoundary</em> attribute is not defined, no restrictions are imposed.
<blockquote class="change-ifc2x4">IFC4 CHANGE  Where rule corrected to accept an <em>IfcOpeningElement</em> for a virtual space boundary.</blockquote>